Patients residing at the intensive care unit are typically monitored for their circulatory or hemodynamic status. This usually includes measurement of arterial blood pressure from either an arterial catheter or a non-invasive blood pressure cuff; and measurement of central venous pressure using a catheter. While measurement of blood pressure is a useful indication for hemodynamic status it does not provide sufficient monitoring for patients where circulation is expected to be most compromised, for example those with shock [1]. In these patients, it is often desirable to measure both pressure and the total blood flow in the circulatory system, known as the cardiac output (CO) [1]. Most clinically importance is the cardiac output from the left ventricle of a normal human being.
The reference technique for measurement of CO is using a thermodilution technique following placement of a Swan-Ganz or pulmonary artery (PA) catheter. PA catheters are placed in the pulmonary circulation via the right side of the heart, making this procedure an invasive technique. The invasive nature of the technique has led to development of a large number of less invasive techniques, ranging from thermodilution performed with catheters placed in the central vein and femoral artery [2] to measurements performed using finger cuffs [3,4]. Less invasive techniques often include a number of extra assumptions and can therefore be less accurate than using a PA catheter.
As measurement of CO can be either invasive or inaccurate and as its measurement is only crucial in patients where circulatory status is compromised, having a method to identify when it is necessary to measure CO would then be advantageous. The present invention generally relates to such systems based upon simulations performed using mathematical models of physiological processes.